(a) Field of the Invention
The invention relates to a focussing transducer for generating ultrasound pulses for the destruction of objects internal to the body, such as concretions and tissue sections, of the kind comprising a spheroidal cup having a mosaic of piezoelectric transducer elements forming the concave surface of the cup, which piezoelectric elements may be energised into oscillation by means of a control apparatus, the transducer having its focus lying on the transducer axis and being alignable on the object in question, and the ultrasound pulses generated being transmissible to the patient's body via a coupling fluid.
(b) Description of the Prior Art
Direct-focussing ultrasound transducers of this kind are known. The DE-A1 27 12 341 discloses an ultrasound transducer of piezoelectric material which is appropriate for examinations by ultrasound in medical diagnostics, in which the transducer body has a concave curvature so that acoustic focussing of the sound waves may be obtained in this manner at a fixed focal point which is determined by the curvature of the transducer. Ring electrodes oppositely situated to an electrode extending throughout the active surface and concentrically applied around a central electrode are situated on the outer surface of the transducer body. The setting of the focal point on the axis of the transducer may be varied to the effect of shortening or lengthening the acoustic focal length, predetermined by the geometrical structure, by energisation of the ring electrodes under variable time-lagging, that is to say up to infinity.
A system organised for the destruction of concretions present in body cavities, of analogous structure to that of the system described in the foregoing, is disclosed, furthermore, in the DE-A1 31 19 295. The characterising feature of this system is a focussing ultrasound transducer which is constructed as a direct sound applicator and with so large an area that the sound output density is so small on the transmission path that tissue damage is prevented, but so great at the acoustic focus that it is adequate for destruction of the concretion present at the focus. In this case too, the division of the transducer surface into rings or matrically assembled individual transducers, serves the purpose to enable the transducer focus to be variably adjustable electronically, according to the phased-array principle.
It is then in the nature of the pulse generation by means of the transducers described that a positive pressure pulse is commonly followed by a negative pulse of greater or lesser magnitude. In this connection, cavitational actions may occur in the negative pressure stage which may have a positive effect in the form of an accelerated destruction, provided this occurs directly in the region of the concretion which is to be destroyed. If however, the cavitational threshold in the interposed tissues or in the adjacent tissues is exceeded during a concretion destroying action, this may lead to undesirable tissue destruction and haemorrhages, especially if the focal point of the transducer is not focussed precisely on the concretion.
As apparent for example from the DE-A1 34 25 992, the aim has already been pursued in the case of lithotripsy, to prevent the appearance of negative pressure pulses or at least reduce the same so far that cavitational actions may be prevented. The steps taken to this end are applicable to a special mechanical structure of the transducer which is intended to ensure that the surge impedance of the material forming the carrying cap for the transducer elements largely corresponds to that of the transducer elements and that the rearward cap surface has no focussing action. Thanks to the absence of reflection established thereby, the deformations of the transducer elements may follow the electrically preset pulse form. Measures of this nature render a transducer so devised particularly appropriate for the destruction of concretions, but they cannot be applied for an aimed or precision destruction of tissue cells, for example in cancer therapy.
The main object of the present invention is to provide an ultrasound transducer which is appropriate for the destruction of concretions as well as of tissue cells and which renders it possible to generate the sound pulses practically at will as regards their amplitude, phase setting, polarity, form and duration.